Cyborg robots, seeking a home world, improving other races by erasing emotions - The Cybermen
The Daleks have been The Doctors longest-running enemy. They were people once, but a thousand-year-long war against the Thals forced them to remake their race into cyborg tanks intent on racial purity and being the superior race of the galaxy. Taking that to it's logical conclusion, the Daleks develop over time technological mastery way beyond anyone's imagination, but dedicated to the cause of wiping out all other inferior life forms.